Pervious concrete is concrete, but it is unique in the sense that is designed to achieve high levels of porosity desired to implement best practices for storm water management. It requires a drier or stiffer consistency. Pervious concrete can have a much broader range of material properties than conventionally placed concrete, especially in instances where additives are used in the mix to control rheology, water hydration levels and overall curing rates.
By definition, pervious concrete is concrete comprising a mixture of cement, large narrow distribution aggregate, little to no sand, and water, having a consistency of a paste allowing it to be compacted with manual or vibratory screeds and typically finished with steel pipe rollers. Pervious concrete can at times incorporate supplemental cementitious materials such as fumed silica, fly ash or blast furnace slag to insure long term performance. It is usually mixed in batches, delivered in trucks with capabilities for quick discharge at the site, and must be finished typically within 1.5 hours from its original batch preparation.
Pervious concrete has generally been used for applications such as parking lots, sidewalks, parks and low volume residential road construction, requiring usually a few inches of crushed gravel bed sub-layer as an essential element to enhance the strength, durability and overall water storage and removal capability.
Conventional poured mass concrete is designed to have the highest fluidity by use of water and additives, the pervious concrete formulations have the same ingredients as conventional concrete, i.e., cement, water, and aggregates, but unlike conventional concrete it is a drier mix, in fact stiff enough to require different techniques to manage application at the job site. Pervious concrete cannot be effectively pumped because of its stiffness and high viscosity and thus ample access to the job site for quick discharge cement trucks is required. Since the success of pervious concrete depends on being able to achieve the proper balance of strength and porosity, the level of pressure applied during the compaction can be achieved with vibratory, laser controlled or manual screeds and finished with steel pipe rollers. To achieve the desired compressive strength, fine mist water spraying and plastic sheeting over the concrete is the usual method for curing this concrete. Current pervious concrete mix can be optimized by adjusting water content and the use of additives such as retardants, hydration stabilizers and rheology modifiers to achieve the best compromise between the full cure properties, void fraction % and workability of the concrete during the application and finishing.
Surfactants decrease the need for excess water, while making the concrete mix pourable as if the added amount of excess water was present. When surfactants are introduced into the concrete, the result is a less porous and somewhat stronger product. The strength increase is attributed to lower water/cement ratio and decrease in porosity.
Attempts have been made to develop mix designs noted as LCE (Low Compactive Effort) to offset normally employed compaction which disrupts the formation of open void structures because of the compressive forces employed to gain the needed design strength. One initial shortfall in the use of LCE mix designs is that they produce lower strength to void ratios, and may therefore only marginally meet the design parameters of the placement. Attempts have been made to affect this shortfall in design performance by incorporating one or more chemical modifiers identified in the trade as High Range Water Reducers, Viscosity/Rheology Modifiers, and Mix Retarders. While a gain in initial strength can be demonstrated by such additives, there is a corresponding lose in permeability which falls below the acceptable design limit.
The prior art provides for compositions, and a process for improving the early strength of cement aggregate products, in a stabilized aqueous emulsion with a surfactant such as alkali metal salts of fatty acids, alkali metal salts of sulfated fatty acids, alkali metal alkyl sulfates, alkali metal alkyl sulfonates, alkali metal aryl sulfonates, alkali metal alkyl lauryl sulfonate, alkali metal salts of alkylated naphthalene, alkali metal salts of lignosulfonic acid, condensation products of ethylene oxide and polyalkylene glycols, fatty acid glycerides, fatty acid amides, polyethylene sorbitol esters of fatty acids, quarternary ammonium halides, sorbitan esters, sulfonated or sulfated fatty acid esters or amides, and sulfonic acid.
The prior art indicates that the introduction of a wax into the composition of water, cement, and aggregate used to produce other concrete products like pervious concrete caused the resulting concrete to have substantially less strength than the concrete product produced from a substantially identical composition without the wax. It is believed that the wax coated the aggregate particles and inhibited the adherence of the cement paste to the aggregate causing the weakening as measured by testing the compressive strength. Therefore, the use of waxes in cement formulations for pervious concrete application is not advisable.